1. Field of the Invention
The present invention relates to a method and system for determining the origin of optical signal quality degradation in optical communication.
2. Description of the Related Art
The point-to-point or point-to-multipoint communication in the optical communication up to the present time bundles signals by applying Optical Time Division Multiplexing (OTDM) to frames offered by a synchronous network.
Such OTDM employs a transmission system such as Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH). The SONET/SDH transmission system defines overhead to perform efficient signal transmission (Reference [1]: ITU-T Recommendation G707), in which the overhead executes parity check called bit interleaved parity between repeaters and between line terminal multiplexer terminals to identify a fault section or to obtain a signal for switching and activating operations.
The signal quality monitoring system such as the SONET/SDH transmission system, however, requires a receiving system corresponding to a bit rate of the signals to be handled, signal format and modulation method (that is, NRZ (Non Return to Zero) or RZ (Return to Zero)). The receiving system comprises a clock extraction circuit, a receiving circuit, a frame detecting circuit and an error detection circuit such as a parity check circuit or collate circuit. Therefore, a single receiving system is not enough to handle a given bit rate, signal format or modulation method. In addition, it is necessary for the conventional optical signal monitoring system to carry out the electric signal processing after converting the optical signal into the electric signal. Accordingly, it is difficult to apply it to the optical amplifier repeater system considering the cost efficiency. For example, even if a network fault is detected, the section between the optical amplifier repeater systems, at which the fault occurs, cannot be identified.
In view of this, it is essential to construct an economical service transfer network that has a sufficient communication capability per service, and can handle a variety of signal formats and signal bit rates. The optical network is extremely promising because it can extend communication capability using OTDM or Wavelength Division Multiplexing (WDM), and has transparency for the signal bit rate, signal format and modulation method.
Thus, as an optical signal quality monitoring system suitable for such an optical network, a method of evaluating an optical signal quality parameter from an amplitude histogram is proposed (Reference [2]: EPC publication number EP0920150A2). FIG. 17 shows the conventional example. An optical signal quality monitoring section 1701 of the conventional example comprises an optical signal amplitude histogram measuring section 1703, an averaged Q-factor parameter evaluation section 1705 and an optical signal quality evaluation section 1707. The optical signal amplitude histogram measuring section 1703 obtains an optical signal amplitude histogram from a optical signal under measurement with a bit rate of f0 (bit/s). The averaged Q-factor parameter evaluation section 1705 evaluates an averaged Q-factor parameter which is the optical signal quality parameter, from the optical signal amplitude histogram. The optical signal quality evaluation section 1707 analyzes the averaged Q-factor parameter to carry out the optical signal quality monitoring.
To meet a sharply growing demand for multimedia services today, it is necessary not only to increase the communication capability of individual services, but also to construct a network capable of efficiently handling a variety of signal bit rates and signal formats associated with video, audio, data and the like. In connection with this, the degradation factors of the optical signals to be monitored are diversified, such as the degradation in the optical signal-to-noise ratio because of the loss of a transmission optical fiber, loss within a transmission terminal and degradation in a light source, and the waveform distortion because of the chromatic dispersion in the transmission optical fiber. Thus, monitoring corresponding to individual degradation factors are especially needed.
However, although the averaged Q-factor parameter in the conventional example is sensitive to the degradation in the optical signal-to-noise ratio and the waveform distortion due to the chromatic dispersion, it cannot discriminate their causes.